Vibrometer



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Oct. l, 1946.

Patented Oct. 1, 1946 VIBROIWETER Donald E. Gillum,

Hermosa Beach, and Charles J. Gallant, Brentwood Heights, Calif., assignors to North American Aviation, Inc., Inglewood, Calif., a corporation of Delaware Application November 29, 1941, Serial No. 421,078

Claims. 1

The .present invention relates generally to electrical vibrometers, the recordings of which are lineal, the amplitude of recording being directly proportional to the amplitude of vibration irrespective of frequency, and has for its main purpose the provision of a displacement measuring instrument of this type which is particularly adapted and suited to the recording of vibrations in aircraft and parts thereof, although not necessarily limited to such use.

In furtherance of the above general purpose, it is an object of the invention to produce a small, lightweight displacement measuring unit, combining high sensitivity, wide frequency range, and a highdegree of calibration stability.

vIt may be said to be a further object to provide an instrument of this type in the nature of a variable capacity pick-up which may be made light in weight whereby the Weight of the instrument itself will have a minimum effect upon its operation; which will eliminate friction as a factor of inefficiency, and which will have a low natural period of oscillation so as to effectively determine the characteristics of relatively low frequency vibrations.

In providing a vibrometer of variable capacitance pick-up type, it is a further object to utilize bodies of rotation in a manner which will cancel out the effect of torsional vibrations, leaving the instrument sensitive to only the lineal components of vibrations being measured.

With these and other objects in mind, the invention proposes in a general Way the use of a pick-up in which statically balanced members are opposed to vibration responsive members in a manner to constitute a variable condenser. This condenser is connected across the terminals of an inductance coil, forming a radio frequency tuned or resonant circuit. It is contemplated in practice that such a coil may be suitably connected to a recording instrument such as a recording oscillograph, as for example by inductively connecting the coil to a source of constant radio frequency, such as an oscillator amplifier, the inductance of the coil and the maximum capacity of the condenser being such that the resonant frequency of the pick-up is the same as the frequency of the oscillator whereby maximum energy is transferred from the oscillator to the pick-up when they are in resonance, and a minimum when they are detuned or out of resonance. If the pick-up is vibrated, causing a. relative movement between the members of the condenser, the resultant change in capacity changes the tuning of its resonant frequency,

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so that the oscillator amplifier translates the variation in capacity into electrical impulses of sufficient magnitude to operate the galvanometer or other responsive element of a recording instrument, such as a recording oscillograph.

The pick-up instrument, constituting the present invention, as hereinafter set forth in greater detail, may be better understood by reference to the accompanying drawing, in which the best mode thus far devised for carrying the same into practical effect, is illustrated mechanically and diagrammatically. In this drawing, which forms a part of this specication,

Fig. 1 is a vertical sectional View through the pick-up instrument.

Fig. 2 is a horizontal sectional View therethrough taken substantially on line 2-2 of Fig. 1.

Fig. 3 is an enlarged fragmentary view partly in elevation and partly in section, of one of the condensers, looking substantially along the line 3 3 of Fig. 2.

Fig. 4 is a detail vertical sectional view through the bearing of one of the rotors, looking substantially along line 4-4 of Fig. 2.

Fig. 5 is a detail horizontal sectional view through the supporting connection of one of the stators, looking substantially along the line 5-5 of Fig. 1.

Fig. 6 is a view illustrating in diagram the electrical hook-up of the stators and rotors.

Referring now to the pick-up as a mechanical instrument responsive to vibrations, and shown in detail in Figs. 1 to 5, a generally fiat mounting base is indicated at I 0. This base member is used as a means of securing the instrument or unit to a test member or frame, indicated at II, and may, therefore, be made to coincide more or less with the test member or frame as may be necessary to properly mount the unit for vibration with, and in accord with, vibrations of the test frame to be measured.

The base member I0 may be secured to the test frame Il by the same screws or other fastening members I2 utilized to hold, on the base member, a shell or cover I3 whose purpose is to protect the working parts of the unit from dust and damage. This shell or cover, it will be noted, has a top opening i4 receiving the upper open end of a socket I5 in which a plug I6 is disposed with a bayonet or other secure holding connection, indi cated at Il, for plugging in such external electrical connections as may be used, for example, to record the vibrations developed by the vibration responsive means.

Secured to, and upstanding from the base member IS, in spaced apart, slightly offset relation, are a pair of brackets I8 and to relatively opposite sides of these brackets, and insulated The offsetting of the brackets I8 relative to each other alines the stator of one bracket with the rotor of the other bracket, as plainly seen in Fig. 2, and each n is normally 'held inhorizontal position with' its larger reeendineiective relation to its relatively opposite stator, each n having at its smaller pivoted end -al depending angular extension 24 connected by a, balance spring 25 to its respective bracket I8 in order to seismically support the ln whose form is preferably ksuchas to place Aits centeri'ofl gravity as nearly as possible toitsfreeend. v AIn vorder to eliminate friction, the posts'2l :of each bracket support adjustable' pivot bearing members v2ias seen best Vin Figfl, having inner recesses ,cooperating with the conical ends of a balance stair Z'I `to form needle pointbearings for the fin pivots.f TheY stallc 2`I is pressed through its respective n fand-through ydamping fluid cups 28 at opposite sidesioi the iin. The cups 28present outer-concave faces spaced-from the adjacent innersurfaces of the posts 2| yto maintain therein, by surface .tension and capil- -larity, a suitable damping fiuid,:indicated at 29, around the needle point bearings-.7:L Y K The manner :of insulating; eachstator from, andlconneeting the same' to,-one ofthe posts 2i voi'zits respective` bracket ,IS- is plainly :seen 'in Figs. 3 and,.aconnect'ingbolt appearingat; 1 The A`two variable 'cond'ensers 'formed n'zthe manner just set forth, are connected in'fparallel, as byjwires 3| `and .132,.respectively Whichmay join thestators. and rotors, across the .terminals of an inductance .coil .tofthus .fzformiaradi'ofrequency. tuned orresonantcircuit. It will-be understoodthat.frelativeamotionbetween thezdamped, balanced rotorii'ln's .22, .23 .and the base, including the stators I9, 28 is.directly proportional tothe @vibrations offthe member lll to Which the base .is attached. Therefore, ras each frespective rotor .andstator constitutes a variable cond'en'ser, lthe vcapacity :of this -con. denser .varies'in direct proportion :to theam'plioude-of vibration. :Were a single .condenser yof this character utilized in .connection lWith ilineal vibrations occurring 'in va Ysingle v'plane at :right anglesto `the length :of .:the stator;iin,zit would be suicient, `butsince zsuchvibrati'ons are seldom in ,practiceiandsince'the'n;is a body fof .rotation ,andis sensitive to :torsional vibration, :the two identical condensers iare .usedln V.the Aopposed manner p-reviouslyset :forth so as to :cancel the effect'of torsional vibrations andleave the Vunit sensitive to onlythe lineal components of 'vibrations.

. As :clearly illustratedin'Figures 1,12, '.3 and 5, the-stators -I9 and 28 .are U-shaped and receive the .larger ends fof the fins 22 and 23 .between their free rends,` such largerendsof th'e flns'extendingsubstantially completely withinithe depth of,A the U VWith such .'construction,v anyvibratory movement of the base lII `in.aidi1ection other thanzin a plane normal to the .baselvvill be translated intc a sidewise movement .of theins 22 and 23. f-However, inasmuch' as 4theeiectiveends 4 of the ns 22 and 23 are disposed Within the U-shaped stators there will be no change in the capacitance of the condenser and therefore no change in a recording instrument to which the vibrometer is connected. With such construction, no variation in capacitance of the condenser :and resultant change in 4a 'recording instrument willbe'h'ad except'in response to vibratory movement in a direction normal to the base -I"l and to the transverse axes of the ns 22 and 23 kin a direction normal to the base Il. It is obvious that as thus described, the vibrometer as proposed thereinhas many advantages in practivvcal use in that'the pick-up requires but one, single conductor shielded, lead and is of small size and light weight -adaptable for. practical use in many instances Where in aircraft vibration measuring, a larger, heavier pick-up would be 'unusable( In addition to its light weight and compactness, the pick-up as set forth herein provides a device by means of which the characterlistics of relatively low -frequency vibrations may be readily and accurately determined by reason of an output proportional to amplitude of vibration independent of frequency. The fact that alternating current ampliers'm'ay be used, lincreases the overall frequency response.

While we have illustrated and described what we now regard as the kpreferred embodiment rof our invention, the construction is, of course,-sub ject to modifications Without departing' from the spirit and scope of our invention. We, .therefore, do'not Wish to restrict ourselves tothe particular form of constructionillustrated and described, but desire to availourselves of rallrmodi cations that may `fall within the scope of the appended'claims.

Having thus described our invention, whatwe claim and desire to Asecure by Letters Patentis:

l. A vbrcmeter comprising a .base .adapted rto be 4placed adjacent a Vibrating ibo'dy, a pair of variable 4condenser-s connectedin 'parallelmounted Ion said base and including 1azpair .of stators and a pair of rotors, and means forseismically supporting the rotors withrespectV tothe'stators so that vibrations transmitted tothe ibase will produce relative movement of therotors the stators and rotors being so .positionedth'at vibrationalforces in a direction perpendicular tothe plane of the -base vcause the capacities to in.- crease or decrease simultaneously While-torsional vibrations will increasethe capacity of one condenser when decreasing `the capacity of theother.

2. A vibrometer'comprising a base adapted to be vplaced adjacent a vibratingfbocly, a pair vof variable condensers connected in'parallel mounted-on saidbase and including a pair ofstators and a-pair of rotors, the rotors Abeing pivoted at one of their ends and `each one `of the pair :of stators being in substantial alignment Withxthe pivot of one :of the rotors, and-means for seismically supporting the rotors with' respect tothe stators so that vibrations transmitted .to the base Will produce relative movement of the rotors the stators -androtorsbeing so positioned that vibrational .iorces-ina direction perpendicular to the plane ofthe base 'cause thecapacities toincrease or decrease .simultaneously While torsional vibrations Will increase the tcapacity :of Aoneacondenser when vdecreasing the `capacity Vof :the other.

3.-.A vibromete'r comprising a base adapted te 'be placed adjacent a vibrating body, -a pair of variable condensers connectedv in parallel mounted .on said base and including ia pair of lstators and 'a'pair of plate-likerotors pivotally mounted at one of their ends, and means for seismically supporting the rotors with respect to the stators so that vibrations transmitted to the base Will produce relative movement of the rotors, the stators and rotors being so positioned that Vibrational forces in a direction perpendicular to the plane of the base cause the capacities to increase or decrease simultaneously While torsional vibrations will increase the capacity of one condenser when decreasing the capacity of the other.

4, A vibrometer comprising a base adapted to be placed adjacent a vibrating body, a pair of variable condensers connected in parallel carried by said base and including a pair of U-shaped stators mounted on the base with the U extending ndrmal to the plane of the base, a, pair of plate-like rotors, each pivotally mounted adjacent one Aof its ends and having its opposite free end extending Within the U of one of the U-shaped stators, and means for seismically supporting the free ends of the rotors in effective relation with respect to the stators so that vibrations transmitted to the base Will produce movement of the free ends of the rotors relative to the stators and resultant change in capacitance of the condenser, the stators and rotors being so positioned that vibrational forces in a direction perpendicular to the plane of the base cause the capacities to increase or decrease simultaneously While torsional vibrations Will increase the capacity of one condenser when decreasing the capacity of the other.

5. A vibrometer comprising a base adapted to be placed adjacent a vibrating body, a pair of variable condensers connected in parallel carried by said base and including a pair of U-shaped stators mounted on the base with the U extending normal to the plane of the base, a pair of plate-like rotors, each pivotally mounted adjacent one of its ends and having its opposite free end extending Within the U of one of the U-shaped stators and each one of the pair of stators being in substantial alignment with the pivot of one of the rotors, and means for seismically supporting the free ends of the rotors in eiective relation with respect to the stators so that vib-rations transmitted to the base will produce movement of the free ends of the rotors relative to the stators and resultant change in capacitance of the condenser, the stators and rotors being so positioned that vibrational forces in a direction perpendicular to the plane of the base cause the capacities to increase or decrease simultaneously While torsional vibrations will increase the capacity of one condenser when decreasing the capacity of the other.

DONALD E. GILLUM. CHARLES J. GAILANT. 

